James Buckley, Simon Bird, Joe Thomas and Blake Harrison of sitcom The Inbetweeners at the premiere of The Inbetweeners Movie in Leicester Square last night. The boys behaved like adults...Photo: WIREIMAGE

The tall, gangly Blake Harrison is too big for our meeting place: a windowless rehearsal room in south London crammed with dozens of props – chairs, tables, bed, radio, TV and books – to give it the feel of a claustrophobic bedsit. This is where the star of Channel 4’s stratospherically popular sitcom The Inbetweeners has been practising for the last three weeks in preparation for his West End theatre debut. Harrison plays Keith, a recovering alcoholic, in Rob Hayes’s new play Step 9 (of 12), and the actual stage – Trafalgar Studios’ black box theatre – is exactly the same dimensions as this impossibly cramped room.

Harrison is clearly worried about how close the audience will be – in the tiny theatre they will surround him on three sides and sit less than a foot away. “I hope the reviewers will have the presence of mind to sit further back,” he says. “I’ll be able to see what they write in their pads!” These nerves are understandable: Harrison’s only previous experience of professional stage acting was a role in a play put on five years ago in a pub in Kennington.

This lack of theatrical experience is unusual for a 26-year-old actor. The generally accepted route to success starts with many years of (badly paid) theatre roles in London followed, if the actor is lucky, by a TV or movie breakthrough. Harrison has followed the same career arc, but in reverse. He landed his first paid acting job shortly after leaving drama school in November 2007, after attending an open audition for a new TV comedy called Baggy Trousers. The producers soon gave it a new name: The Inbetweeners. He says the audition felt like a "cattle market" but he was asked to come back and eventually won the part of the dim-witted and hilariously gullible Neil.

Created by The 11 O’Clock Show writers Damon Beesley and Iain Morris, The Inbetweeners follows four teenagers at a suburban comprehensive as they experience all the things that come with being boys at that age: a lack of girls and money, negative street cred and, of course, an infuriating inability to get served at their local.

There have been three series of The Inbetweeners so far, and the series has been nominated for two Baftas and won an award for Outstanding Contribution to British Comedy in 2011. In 2009, when the TV series was at the peak of its popularity, 1.2 million viewer tuned in, making The Inbetweeners the most-watched programme that year on E4. Last year, the spin-off movie became the biggest grossing British comedy of all time, and rocketed its leading men to unlikely stardom.

Harrison’s Inbetweeners character has been such a big part of his life for the last five years, he can instantly transform his face to give Neil’s brilliantly gormless stare (a look Harrison would regularly use before The Inbetweeners came along to wind up his brother). But in person he’s very different. Sharp, funny and garrulous, he was amazed when he got the part after assuming during the auditions that he was being considered for the role of hopeless romantic Simon, who is played by Thomas. “[At school] I was the lad who constantly wanted a long-term relationship,” he says. “I’d start going out with someone and then after a month I’d get dumped. I never thought I’d end up playing Neil.”

Now, with The Inbetweeners film and a leading part in the British-American TV comedy The Increasingly Poor Decisions of Todd Margaret on his CV, Harrison’s changing tack dramatically with Step 9 (of 12). The title references the 12 steps to recovery famously laid out by Alcoholics Anonymous and the play's themes cover addiction, foster parenting and various forms of abuse. There are some laughs, however. “The humour is realistic,” says Harrison, “and Keith is funny. But he uses it in situations when he really shouldn’t, which makes people feel uncomfortable.”

Harrison had seven years of performing in plays at drama school, but theatre acting is not what he’s now used to. Leaning the lines is harder, he says. “With film you learn a day’s worth of lines and then forget about them. Theatre is a different discipline. There are still moments in rehearsal where I need a little prompt.”

Self-discipline is one attribute that wasn’t necessarily required on the set of The Inbetweeners, where the cast evidently share their characters’ love of childish dares. “Having to eat an entire bag of sugary Haribo in two minutes just before a scene once made Joe Thomas go green,” Harrison remembers wistfully.